1. Field of the Invention
The present invention relates to a current injection type electromagnetic wave oscillator. In particular, the present invention relates to a current injection type oscillator in a frequency region ranging from millimeter wave to terahertz wave (30 GHz to 30 THz). Still more particularly, the present invention relates to a current injection type oscillator having a resonant tunneling diode structure.
2. Description of the Related Art
In recent years, nondestructive sensing techniques using an electromagnetic wave having frequencies of at least a part of a frequency region ranging from millimeter-wave to terahertz-wave (30 GHz to 30 THz) (hereinafter, simply referred to as terahertz wave) have been developed. Examples of such techniques, which have been developed in application fields of the electromagnetic wave of this frequency band, include an imaging technique using a safe fluoroscopic apparatus in place of an X-ray fluoroscope, and a spectral technique for obtaining an absorption spectrum or complex dielectric constant of a material to examine physical properties such as a bonding state. In addition, a technique for analyzing biomolecules, a technique for evaluating a carrier concentration or mobility, and the like have also been developed.
Examples of terahertz wave generating units include a unit for emitting a femtosecond laser beam to a photoconductive device to generate a pulse and a parametric oscillation unit for emitting a nanosecond laser beam to a nonlinear crystal to generate a specific frequency. However, each of the units is based on photoexcitation, so there is a limit on a reduction in size or power consumption.
Therefore, for example, a structure using a quantum cascade laser and a structure using a resonant tunneling diode (resonant tunneling diode type) have been studied for the current injection type device that operates in the terahertz wave region. In particular, the latter resonant tunneling diode type is expected for a device that operates at around 1 THz at room temperature (see Japanese Journal of Applied Physics, Vol. 44, 2005, pp.7809-7815). The resonant tunneling diode type typically includes a quantum well with a lattice-matched system InGaAs/InAlAs which is formed on an InP substrate by epitaxial growth. In order to thin a barrier layer to improve current density, an AlAs structure has been studied. With respect to a voltage-current (I-V) characteristic and an oscillation characteristic of the AlAs structure, as illustrated in FIG. 12, a negative resistance is exhibited and an oscillation in the vicinity of a region corresponding to the negative resistance is observed.
However, in this case, the oscillation output is limited by a gain resulting from the negative resistance in an active layer and an injectable current density. Therefore, in order to obtain a high power output, it is necessary to improve the structure of the active layer and the structure of the resonator. That is, it is desirable to increase the peak current density and the gradient in the negative resistance region (negative differential conductance) shown in FIG. 12.
An example of the disclosed structure for improving the current density to increase the gradient in the negative resistance region is a structure for improving the current density using a strain system (compressive strain), in which an In composition of an InGaAs well layer is increased (see Japanese Patent Application Laid-open No. 2004-200286).
Incidentally, in order to reduce propagation loss of a transmission line of a resonator in an oscillator, it is desirable to thicken the active layer. Therefore, a method of providing a triple barrier structure to thicken the active layer without reductions in gain and current density is expected.
However, when the triple barrier structure is to be provided using the strain well structure described in Japanese Patent Application Laid-open No. 2004-200286 to improve the characteristics including an oscillation output of the oscillator, because there is a critical film thickness due to the use of the strain well structure, the thickness cannot be increased above a certain level. Therefore, it is not easy to reduce a resonator loss using the resonant tunneling diode structure and to improve the current density and the gain to thereby increase the oscillation output.